HLSL: allow promotion from 1-vector types to scalars, e.g, float<-float1

HLSL: allow promotion from 1-vector types to scalars, e.g, float<-float1

Previously, an error was thrown when assigning a float1 to a scalar float,
or similar for other basic types.  This allows that.

Also, this allows calling functions accepting scalars with float1 params,
so for example sin(float1) will work.  This is a minor change in
HlslParseContext::findFunction().